The present invention relates to an image forming apparatus equipped with a driving force transmission mechanism, and a process unit used by such an image forming apparatus.
Conventionally, a driving system for driving a photoconductive drum, as an image bearing member, of an image forming apparatus such as a copying machine, a printer, or the like comprises: a driving gear, which receives the driving force from the motor of the main assembly of the image forming apparatus, and transmits the driving force; and a gear shaft, which is coaxial with the driving gear, and rotates with the driving gear. There are two methods for driving a photoconductive drum with the use of such a driving system: a driving method which employs a driving shaft with a coupling, and a driving method which employs a driving shaft without a coupling. FIG. 12 shows the rough concept of a driving method which employs a driving shaft without a coupling.
Referring to FIG. 12, a gear shaft 13 directly connected to a drive gear 12 is put through a photoconductive drum 80, and is used as the drum shaft for the photoconductive drum 80. In this case, the photoconductive drum 80 is supported so that it rotates with the gear shaft 13. With this driving method, the rotation of the driving gear 12 for transmitting the driving force of a motor 11 is directly transmitted to the photoconductive drum 80.
FIG. 13 shows the rough concept of the driving method which employs a driving shaft with a coupling. In this case, the gear shaft 13 is indirectly connected to the photoconductive drum 80, with the interposition of a coupling 23, and the driving force from the motor 11 is transmitted to the photoconductive drum 80 through the gear shaft 13 and coupling 23.
Comparing the two driving methods, the method which employs a coupling is superior in terms of cost. Further, in the case of an image formation system constructed around a photoconductive drum, a photoconductive drum has come to be integrally disposed, along with a single or a plurality of processing apparatuses, such as a developing apparatus, in a cartridge (process cartridge). Consequently, it has become considered important how easy a process cartridge is to mount into, or dismount from, the image forming apparatus main assembly. This has called attention to various couplings, that is, devices for connecting two shafts to transmit a driving force from one shaft to the other. Among various couplings, couplings such as the one shown in FIG. 14 which comprise a combination of a female type coupler and a male type coupler, and in which driving force is transmitted through the engagement between the two couplers, have begun to attract special attention because of their superiority in terms of driving force transmission performance. The couplings such as the one shown in FIG. 14 have come to be widely used, because of their advantage in driving force transmission performance.
However, a coupling based driving method is inferior to a direct driving method in terms of driving force transmission accuracy; in other words, there is a concern that a coupling based driving method suffers from the problem regarding the angle at which two shafts are connected to each other by a coupling, the problem regarding how two shafts are aligned with each other by a coupling, or the like problems. When an image forming apparatus suffers from the problems described above, an image becomes misaligned relative to a recording medium, degrading image quality. This problem is particularly conspicuous in an image forming apparatus which forms a color image on recording medium by placing a plurality of images different in color, in layers.
As a coupling for solving the above described problems, there is a coupling comprising a combination of a female type coupler and a male type coupler, at least one of which is in the form of a polygonal piller. FIG. 15 shows an example of such a coupling comprising a female type coupler with a hole in the form of a polygonal pillar, and a male type coupler with a projection in the form of a polygonal pillar.
Referring to FIG. 15, a gear shaft 13, that is, one of the components of the driving system I of the apparatus main assembly, is provided with a female type coupler 14, which is attached to one end of the gear shaft 13, and is provided with a twisted hole 50, the cross section of which is in the form of an equilateral triangle.
On the other hand, the photoconductive drum 80, or a cylindrical member which constitutes one of the components of the system to be driven by the driving system I, is provided with a twisted projection 10, which is the same in twist angle as the twisted hole of the female type coupler 50, and the cross section of which is in the form of an equilateral triangle. A male type portion 53 with the projection 10 in the form of a twisted pillar with a cross section in the form of an equilateral triangle doubles as a drum flange 80. Although the projection 10 is in the form of an equilateral triangular pillar, here, it may be in the form of a polygonal pillar, the cross section of which is not in the form of an equilateral triangle. When one of the lateral walls of the hole of the female type portion is in contact with one of the lateral walls of the male type portion, the interface between the two members forms a twisted line 60. Therefore, as a rotational force is applied to the female type portion while the two members are in engagement with each other, the male type portion, that is, the member on the photoconductive drum 80 side is pulled into the female type portion, or the member on the apparatus main assembly side, being accurately positioned relative to the apparatus main assembly. As a result, the two shafts are precisely connected by this effect, that the male type portion is pulled into the female type portion, and the play between the photoconductive drum 80 and apparatus main assembly in terms of the thrust and radial directions of the photoconductive drum 80 is eliminated.
FIG. 16 shows the cross sections of the female and male type portions 14 and 53 in engagement with each other, at a plane perpendicular to the axial lines of the two members. In FIG. 16(a), the solid line represents the cross section of the female type portion 71, or the driving side, with the hole 50, the cross section of which is in the form of an equilateral triangle, and the broken line represents the projection 10, or the driven side, the cross section of which is in the form of an equilateral triangle. Under the condition depicted by FIG. 16(a), although the projection 10 is in the hole 50, the edges of the projection 10 are not in contact with the corresponding lateral walls of the hole 50, and therefore, there is play between the two members, and the axial line C1 of the female type portion does not coincide with the axial line C2 of the male type portion. However, as rotational force is applied to the female type portion 71, the three lateral walls of the hole 50 contact the three edges of the projection 10, one for one, automatically causing the axial lines C1 and C2 to coincide with each other, as shown in FIG. 16(b). Under the condition depicted by FIG. 16(b), the driving force applied to the female type portion 71 is transmitted as forces F by the interfaces (contact points) between the female and male type portions.
As described above, a coupling which comprises a combination of a female type portion and a male type portion, and in which the male type portion is engaged into, and then is placed in contact with, the female type portion, makes it possible to connect two shafts while automatically aligning the two shafts, accurately positioning them, and eliminating the play. Therefore, such a coupling is considered effective as a member for the connection between the photoconductive drum of an image formation unit in the form of a cartridge, and the image forming apparatus main assembly. Further, a coupling, in which the hole of the female type portion and the projection of the male type portion are in the form of a twisted polygonal pillar, provides the effect that a photoconductive drum is pulled toward the image forming apparatus main assembly in terms of their axial directions, in addition to the above described effects.
The above described coupling, however, suffers from the problem that as the driven system is subjected to a force other than the force transmitted from the driving system, the contact between the two systems is disturbed. For example, as a photoconductive drum comes into contact with an intermediary transfer member, the photoconductive member is subjected to such force that is applied to the photoconductive member in the circumferential direction of the photoconductive member, and this force disturbs the contact between the two members of the above described coupling. More specifically, two shafts to which the female and male type portions of the aforementioned coupling are attached one for one are kept aligned with each other by the contact between the two members of the coupling, and therefore, as the contact between the two members is disturbed, the two shafts become misaligned with each other, causing an image to be misaligned with the recording medium, as the image is formed. This has been a significant problem.
Thus, the primary object of the present invention is to provide a driving force transmission mechanism, which assures that even when the contact between the female and male type portions of a coupling is disturbed by a force other than the force transmitted to the driven system from a driving system, the normal contact is maintained between the female and male type portions of the coupling, so that the cylindrical member of a driven system remains properly supported, and the accuracy of the alignment between the driven and driving systems is maintained regardless of the size of the play between the female and male type portions when they are in engagement with each other.
According to an aspect of the present invention, there is provided a drive transmission apparatus comprising a first coupling portion having a polygonal shape; a second coupling portion having a hole portion which has a cross-sectional configuration larger than the first coupling portion, the hole portion being engageable with the first coupling portion; and a center shaft provided on the first coupling or the second coupling, the center shaft penetrating the other one of the first and second coupling.
According to another aspect of the present invention, there is provided an image forming apparatus comprising a photosensitive member; charging means for charging the photosensitive member; image forming means for forming an electrostatic image on the photosensitive and charged by the charging means; developing means for developing the electrostatic image; transferring means for transferring the image developed by the developing means onto a recording material; a driving source; a driver for transmitting a driving force from the driving source to the photosensitive member; a first coupling portion having a polygonal shape; a second coupling portion having a hole portion which has a cross-sectional configuration larger than the first coupling portion, the hole portion being engageable with the first coupling portion; and a center shaft provided on the first coupling or the second coupling, the center shaft penetrating the other one of the first and second coupling; wherein the photosensitive member has one of the first coupling portion and the second coupling portion, and the driver as the other coupling portion.
According to a further aspect of the present invention, there is provided a process unit which is detachably mountable to an image forming apparatus having a driving portion, the process unit including process means actable on the photosensitive member, the process unit comprising a first coupling portion having a polygonal shape and engageable with the driving portion of the main assembly of the apparatus; a hole portion engaged with a center shaft penetrating an engaging portion between the first coupling portion and the driver.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.